The invention relates to a composite body formed from two or more parts, at least one of which consists of an oxidic material which may be bonded to one or more parts of a metallic material or a mixture thereof with a metal oxide (cermet), the oxidic material being chosen from densely sintered aluminium oxide (dga), rare earth perowskites of the type ABO.sub.3, wherein A represents La, Ce, Pr, Nd or a mixture thereof and B represents: Al, Sc, Lu or a mixture thereof, garnets of the type A.sub.3 Al.sub.5 O.sub.12, wherein A is Mg, Li or Y, or sapphire, and the metallic part being chosen from tantalum, niobium, tungsten, molybdenum, alloys having one of these metals as the main component and alloys having iron, nickel or cobalt as the main component, molybdenum-silicon alloys, titanium boride and titanium nitride, which parts are bonded together in a gas-tight and vacuum-tight manner with a bonding material, the bonding material containing aluminum oxide and at least one oxide of a rare earth metal.
The invention particularly relates to a gas discharge lamp the envelope of which consists of the above-mentioned densely sintered aluminum oxide, which lamp comprises an electrode feed-through of niobium and contains sodium vapour or a metal halide vapour as the gas filling during operation. Densely sintered aluminum oxide is a material which is perfectly suitable for this purpose as it is resistant to the action of sodium vapour and to the action of metal halides, even at temperatures as high as 1200.degree. C.
German Patent Specification 17 71 143 (U.S. Pat. No. 3,926,603) discloses a material for bonding bodies of densely sintered aluminum oxide to each other and to metal bodies, this material having the following composition expressed in % by weight:
CaO--24-50 PA1 Al.sub.2 O.sub.3 --35-57.5 PA1 MgO--0-12 PA1 BaO--0-16 PA1 Y.sub.2 O.sub.3 --0-10 PA1 B.sub.2 O.sub.3 --0-9 PA1 ZrO.sub.2 --0-17,5 PA1 Li.sub.2 O--0-3 PA1 Al.sub.2 O.sub.3 --5-70 PA1 2&lt;x&lt;4--2-70 PA1 Y.sub.2 O.sub.3, Gd.sub.2 O.sub.3, Tb.sub.2 O.sub.3, Dy.sub.2 O.sub.3, HO.sub.2 O.sub.3, Er.sub.2 O.sub.3, Eu.sub.2 O.sub.3, Pr.sub.2 O.sub.3, Ce.sub.2 O.sub.3, Sm.sub.2 O.sub.3, Tm.sub.2 O.sub.3, Yb.sub.2 O.sub.3, Lu.sub.2 O.sub.3, La.sub.2 O.sub.3 and Nd.sub.2 O.sub.3 --10-80. PA1 Ti.sub.2 O.sub.x --40-50 PA1 Ln.sub.2 O.sub.3 --15-25 and PA1 Al.sub.2 O.sub.3 --30-40, PA1 Ti.sub.2 O.sub.x --15-25 PA1 Ln.sub.2 O.sub.3 --10-20 and PA1 Al.sub.2 O.sub.3 --60-70
the sum (B.sub.2 O.sub.3 +ZrO.sub.2 +Li.sub.2 O) being between 3 and 20% by weight.
This vitreous composition is devitrified, preferably after having been applied in the sealing seam, by a thermal treatment to a fine-crystalline product which especially in this state is resistant to attack by sodium vapour at temperature up to approximately 900.degree. C.
Because of the fact that a white light-emitting sodium vapour discharge lamp has been developed in which a sodium pressure of 300-600 torr is used, which is realised by a higher temperature of the coldest spot in the discharge lamp, the need has now arisen for a material which is resistant to sodium and sodium aluminate up to temperatures of approximately 1000.degree. C. and which is suitable for use as a sealing material at a temperature lower than 1750.degree. C., preferably lower than 1650.degree. C.
It has been found that the prior art bonding material on the basis of alkaline earth aluminate is not sufficiently resistant to the corrosive action of sodium atoms and sodium ions. In the above-mentioned, novel discharge lamp the bonding material is exposed at a temperature of 850.degree.-950.degree. C. to sodium vapour having a vapour pressure of 300-600 torr and to liquid sodium in the form of an amalgam. In addition, the bonding material is also exposed to contact with sodium ions originating from a sodium-.beta.-aluminate layer formed during operation. All this results in the formation of unwanted compounds by ion exchange which shorten the life of the bond and consequently of the lamp to an impermissible extent.